Experimental Determination of the 24Mg I (3s3p)3P2 Lifetime

We present the first experimental determination of the electric-dipole forbidden (3s3p)P-3(2) -> (3s(2))S-1(0) (M2) transition rate in Mg-24 and compare to state-of-the-art theoretical predictions. Our measurement exploits a magnetic trap isolating the sample from perturbations and a magneto-optical trap as an amplifier converting each P-3(2) -> S-1(0) decay event into millions of photons readily detected. The transition rate is determined to be (4.87 +/- 0.3) X 10(-4) s(-1) corresponding to a P-3(2) lifetime of 2050(-110)(+140) sec. This value is in agreement with recent theoretical predictions, and to our knowledge the longest lifetime ever determined in a laboratory environment.